This invention relates to receiving data signals and to digital filter circuits that are of use especially in the receiving of data signals.
One application where data signals have to be received is computer networks, for example, token ring networks. In one token ring network, it is required to receive packets of data which are up to 18 kbit in length. Even if they are transmitted at a constant rate, the symbols used to represent the data on a communications channel do not arrive at the receiver at a constant rate. This effect is known as jitter. One of the sources of jitter is that different symbols can have different frequencies in their spectra and so might travel along the channel at different speeds.
In network interfaces, the conventional method used to cope with jitter is for the receiver to employ an analogue phase locked loop to maintain a clock signal in a particular phase relationship with the symbols of the incoming data signal. That clock signal is then used to actuate a latch to read the data from the data signal.
An analogue phase locked loop usually comprises a voltage-controlled oscillator providing the clock signal, a phase comparator providing an indication of the difference in phase between the clock signal and the data signal, and a loop filter responsive to that indication controlling the voltage supplied to the oscillator.